Recently, development of a current driving light emitting element such as an organic EL display and FED has been actively promoted. Particularly, the organic EL display attracts attentions as a display for a portable device such as a mobile phone and PDA (personal digital assistance) since the organic EL display can emit light with a low voltage and low power consumption.
An arrangement of a pixel circuit of the organic EL display of Japanese National Publication of Translated Version No. 514320/2002 (Tokuhyo 2002-514320)(Publication date: May 14, 2002) is illustrated in FIG. 8.
A pixel circuit 300 illustrated in FIG. 8 includes: a driving TFT 365; switching TFTs 360, 370, and 375; capacitors 350 and 355; and an organic EL element (OLED) 380. Each of the four TFTs (thin film transistors) is a p-channel type.
The driving TFT 365, the switching TFT 375, and the organic EL element 380 are connected in series between a power source line (+VDD line) 390 and a common cathode (GND line) so that the driving TFT 365 is positioned on the side of the power source line 390. The switching TFT 360 and the capacitor 350 are connected in series between a gate terminal of the driving TFT 365 and a data line 310 so that the capacitor 350 is positioned on the side of the driving TFT 365. Further, the switching TFT 370 is provided between the gate terminal and a drain terminal of the driving TFT 365, and the capacitor 355 is provided between the gate terminal and a source terminal of the driving TFT 365.
A gate terminal of the switching TFT 360 is connected to a select line 320, and a gate terminal of the switching TFT 370 is connected to an auto zero line 330, and a gate terminal of the switching TFT 375 is connected to an illuminate line 340.
In the pixel circuit 300, the auto zero line 330 and the illuminate line 340 becomes low at a first period, so that the switching TFTs 370 and 375 become ON. This causes the drain terminal and the gate terminal of the driving TFT 365 to have the same potential. At this time, the driving TFT 365 becomes ON, so that a current comes to flow from the driving TFT 365 toward the organic EL element 380. At this time, the data line 310 is made to have a reference potential, and the select line 320 is made low, and the other terminal of the capacitor 350 (i.e., a terminal on the side of the switching TFT 360) is made to have the aforementioned reference potential.
At a second period, the illuminate line 340 is made high, so that the switching TFT 375 becomes OFF. As a result, a potential of the gate terminal of the driving TFT 365 gradually rises, and the driving TFT 365 becomes OFF when the potential has a value (+VDD+Vth) corresponding to a threshold voltage (Vth; Vth is a gate-source voltage and has a negative value) of the driving TFT 365.
At a third period, the auto zero line 330 is made high, so that the switching TFT 370 becomes OFF. As a result, a difference between the gate terminal potential of the switching TFT 370 and the reference potential is stored in the capacitor 350 at this time. That is, the gate terminal potential of the driving TFT 365 has a value (+VDD+Vth) corresponding to a threshold state (a state in which the gate-source voltage becomes the threshold voltage Vth) when a potential of the data line 310 is the reference potential. Further, when the potential of the data line 310 varies from the reference potential, a current corresponding to the potential variation flows to the driving TFT 365 regardless of the threshold voltage of the driving TFT 365.
Such a desired potential variation is given to the data line 310, and subsequently the select line 320 is made high so as to turn the switching TFT 360 OFF. This keeps the gate terminal potential of the driving TFT 365 as an inter-terminal voltage of the capacitor 355, and a selection period of the pixel circuit 300 is ended.
An example where the potential is set in this manner is illustrated in FIG. 9 for example. In FIG. 9, the reference potential is Vpc, and the data line 310 has a potential Vdata as a potential having varied from the reference potential Vpc.
As described above, with the pixel circuit 300 of FIG. 8, it is possible to set a current value, outputted from the driving TFT 365 to the organic EL element 380, regardless of the threshold voltage of the driving TFT 365.
A pixel circuit illustrated in FIG. 10 is described in Japanese Unexamined Patent Publication No. 351401/2002 (Tokukai 2002-351401)(Publication date: Dec. 6, 2002). The pixel circuit 200 of FIG. 10 includes: a driving TFT 202; switching TFTs 201, 203, 204, and 205; capacitors 251 and 252; and an organic EL element (OLED) 253. Each of the five TFTs is a p-channel type.
The driving TFT 202, the switching TFT 204, and the organic EL element 253 are connected in series between a power source line (+VDD line) 271 and a common cathode (GND line) so that the driving TFT 202 is positioned on the side of the power source line 271. Further, the switching TFT 205 is connected to the organic EL element 253 in parallel.
The switching TFT 201 and the capacitor 251 are connected in series between a gate terminal of the driving TFT 202 and a data line 272 so that the switching TFT 201 is positioned on the side of the data line 272. The switching TFT 203 is provided between the gate terminal and a source terminal of the driving TFT 202.
A gate terminal of the switching TFT 201 is connected to a select line 281, and a gate terminal of the switching TFT 203 is connected to a control signal line 283, and a gate terminal of the switching TFT 204 is connected to a control signal line 284, and a gate terminal of the switching TFT 205 is connected to a control signal line 285.
In the pixel circuit 200, as illustrated in FIG. 11, the control signal lines 283, 284, and 285 become low at a first period (time t3 to time t4), so that the switching TFTs 203, 204, and 205 become ON. This causes a drain terminal and the gate terminal of the driving TFT 202 to have the same potential. As a result, the driving TFT 202 becomes ON, so that a current flows from the driving TFT 202 toward the common cathode. At this time, a current depending on a ratio between an impedance of the switching TFT 205 in an ON state and an impedance of the organic EL element 253 flows to each of the switching TFT 205 and the organic EL element 253. Further, the data line 272 is made to have a reference potential Vpc, and the select line 281 is made low at this time, so that the switching TFT 201 becomes ON, and the other terminal of the capacitor 251 (i.e., a terminal on the side of the switching TFT 201) has the reference potential Vpc.
At a second period (time t4 to time t5), the control signal line 284 is made high, so that the switching TFT 204 becomes OFF. As a result, the gate terminal potential of the driving TFT 202 gradually rises, so that the driving TFT 202 becomes OFF when the gate terminal potential has a value (+VDD+Vth) corresponding to a threshold value (Vth; Vth is a gate-source voltage and has a negative value) of the driving TFT 202.
At a third period (time t5 to time t9), the control signal line 283 is made high, so that the switching TFT 203 becomes OFF. As a result, a difference between the gate terminal potential of the switching TFT 203 and the reference potential Vpc is stored in the capacitor 251 at this time. That is, the gate terminal potential of the driving TFT 202 has a value (+VDD+Vth) corresponding to a threshold state (a state in which the gate-source voltage becomes the threshold voltage Vth) when a potential of the data line 272 is the reference potential Vpc. Further, if the potential of the data line 272 varies from the reference potential Vpc to a potential Vdata, a current corresponding to the potential variation flows to the driving TFT 202 regardless of the threshold voltage of the driving TFT 202.
Such a desired potential variation is given to the data line 272, subsequently the select line 281 is made high so as to turn the switching TFT 201 OFF. This keeps the gate terminal potential of the driving TFT 202 as an inter-terminal voltage of the capacitor 252, and a selection period of the pixel circuit 200 is ended.
By using the pixel circuit 200 of Fig. 10 in this manner, it is possible to set a value of a current, outputted from the driving TFT 202 to the organic EL element 253, regardless of the threshold voltage of the driving TFT 202. Also, by reducing the switching TFT 205's impedance at the time when the switching TFT 205 is ON, it is possible to suppress a current flowing from the driving TFT 202 to the organic EL element 253 at the first period.
With the pixel circuit 300 of FIG. 8, it is possible to flow a desired current to the organic EL element regardless of the threshold voltage of the driving TFT 365. However, a current flows from the driving TFT 365 to the organic EL element 380 during the first period, so that the organic EL element 380 emits light. Originally, the first period is a period in which no light is emitted and no current is flown to the organic EL element 380, so that the foregoing arrangement raises such a problem that a contrast is lowered and the organic EL element 380 is deteriorated.
This is the same as in the pixel circuit 200 of FIG. 10. That is, by turning ON the switching TFT 205 provided in parallel to the organic EL element 253 at the first period, it is possible to suppress the current flowing to the organic EL element 253. Theoretically, if the impedance at the time when the switching TFT 205 is ON can be made zero or the impedance of the organic EL element 253 can be made infinite, it is possible to prevent a current from flowing to the organic EL element 253 at the first period. However, it is difficult to make the switching TFT 205's impedance zero at the time when the switching TFT 205 is ON, and the impedance of the organic EL element 253 is finite, so that a current depending on a ratio between the impedance of the organic EL element 253 and the impedance of the switching TFT 205 flows to each of the organic EL element 253 and the switching TFT 205. Thus, also in case of using the pixel circuit 200 of FIG. 10, the contrast is inevitably lowered and the organic EL element is inevitably deteriorated.